natland



March 31, 1964 M. L. NATLAND APPARATUS FOR HEATING AN UNDERGROUND FORMATION Filed Feb. 8, 1957 INVENTOR MANLEY L. N ATLAND ATTORNEYS Z wr I March 31, 19 M. L. NATLAND 3, 7, 9

APPARATUS FOR HEATING AN UNDERGROUND FORMATION Filed Feb. a. 1957 5 Sheets-Sheet I5 INVENTOR MANLEY L. NATLAND ATTORNEYb 3,127,319 APPARATUS F R HEATING AN UNDERGROUND FGRMATIUN Manley L. Natland, Rolling Hills, Calii, assignor to Rich'fieid Oil Corporation, Los Angeles, Calif., a corporation of Delaware Filed Feb. 8, 1957, Ser. No. 639,001 Claims. (Cl. 1176-17) This invention pertains to an apparatus for heating an underground formation, for instance to improve the recovery of oil from a subsurface oil-bearing stratum. More particularly, this invention relates to an apparatus including a nuclear reactor which is positioned in a well, for the purpose of applying heat to a desired subsurface stratum.

Numerous methods for heating oil-bearing sands have been proposed such as the circulation of a heated fluid through the bore hole; and the generation of heat adjacent or within the stratum by chemical means, by the suspension of electrical heaters in the well, and by the ignition of oil or gas in the lower portion of the well bore or in an adjacent stratum. However, all of these methods are attended with certain difficulties such as the loss of heat to strata overlying the desired location of heating in the circulating fluid method. When directly firing oil or gas in the bore hole, provision must be made for piping fuel and air to the bottom of the well and frequently an igniting means must be strung with the well tubing. The ignition devices are difiicult to operate and are unreliable, particularly when the well is under substantially elevated temperatures and pressures. The chemical methods for heating underground strata are difficult to control from the earths surface over substantial periods of time and the high voltages necessary to obtain the desired temperatures by electrical heating can give rise to insulation difiiculties at the temperatures encountered at substantial depths.

According to the present invention I have provided an apparatus for applying any desired degree of heat to an underground stratum, e.g., an oil-bearing formation, said apparatus being compact and capable of control. The apparatus is useful in any method where heating of a well bore is desired, such as to improve recovery of oil from the bore by a reduction in viscosity, to heat an input well in a thermal recovery process where production is taken from one or more output wells, etc. More particularly, I have provided a nuclear reactor which can be positioned in a well bore hole remotely or at any substantial distance from the earths surface, the reactor being associated with control and temperature indicating means at the surface of the earth so that the rate of fission and ultimately the temperature within the bore hole can be regulated, usually in the range of from about 300 F. to 2000 F. and pref erably from about 400 F. to 1500 F.

It is generally known that the bombardment of fissionable isotopes such as U 239, 233 and U 235 with what are termed thermal neutrons will result in the fission of the isotope nuclei accompanied by a large production of energy and the release of about two fast or high energy level neutrons which can in turn beslowed down to the thermal equilibrium of the surrounding medium thereby eifecting additional fission. If fissionable material is present in at least a critical mass and the thermal neutron density reaches a minimum critical level, a self-sustained chain reaction will be established. Since the neutrons produced in the fission of the nuclei have very high energy levels and consequently will rebound from the fissionable nuclei rather than be absorbed therein they must pass through some substance which will moderate, or slow them down to the thermal equilibrium of the system before coming in contact with other fissionable nuclei. This 3,127,319 Patented Mar. 31, 1964 is generally accomplished by locating a moderating material such as graphite, deuterium oxide or beryllium around and between the particles of fissionable material thereby necessitating the passage of a substantial portion of the fast neutrons through this substance before they enter or contact a fissionable nuclei. The theories and formulas concerning these relations are given in detail in the patent to Fermi et al., US. No. 2,708,656, and need not be dwelled upon at this point. In addition to the slow or thermal neutron reactors there are also fast neutron reactors which are capable of being controlled.

The fission of the nuclei produces large amounts of energy in the form of heat. In order to make practical use of the nuclear reactor and the subsequent production of heat it has been necessary to devise ways and means of controlling the rate of fission. Generally this has been accomplished by controlling the neutron flux or density within the reactor by one or a combination of the following means: (1) variations in the concentration of the nuclear fuel, (2) neutron absorbers or control rods, (3) fuel geometry and (4) variations in neutron reflector positions.

The apparatus of my invention provided for heating an underground formation includes a nuclear reactor inserted into a well bore and generally positioned opposite the stratum which it is desired to heat. This reactor has means for controlling the fission reaction by manipulation from the earths surface, and I provide means to indicate at the earths surface the temperature of the reaction. In one form the nuclear reactor of my invention is of the type wherein the rate of fission and ultimately the temperature can be increased or decreased by introducing or withdrawing from the reaction chamber a portion of the fissionable material, the movable portion of the fissionable material being separated from the stationary portion by a neutron moderating material. Other means for controlling the reaction could, of course, be provided, for example the position of the neutron moderating material could be changed to expose more or less of it to the neutrons in a reaction chamber containing fissionable material in an amount at least equal to the critical mass, thus increasing or decreasing the number of neutrons which are slowed to aiford greater or lower temperatures as the case may be. Alternatively, the reaction might be controlled by varying the amount of neutron absorbing material in the reaction chamber containing the critical mass of fissionable material. In fact, any of the various types of nuclear reactors couldbe employed. including the fast neutron reactors.

More specifically my invention may be best understood by reference to the following drawings wherein:

FIGURE 1 is a view partially in cross-section of one embodiment of my nuclear reactor shown in relation to an oil'well bore, the reactor being oppositea bare oil-bearing stratum below a cemented casing;

FIGURE 2 is a cross sectional view taken throughv line 2-2 of FIGURE 1;

FIGURE 3 is a second embodiment of my. reactor combined with an oil pump within a cased well. The reactor and pump are placed opposite a' perforated, cemented liner adjacent an oil bearing sand;

FIGURE 4 is a view partially in cross-section of' the reactor of FIGURE 3- in operative position; and

FIGURE 5 is a cross-sectionalview through line-S-S of FIGURE 4.

FIGURE 6 is still another embodiment: partially in section of my invention shown inrespect to an oil well' bore hole.

FIGURE 7 is a cross-sectional view through line-7 7 of' FIGURE 6.

Referring to FIGURES 1 and' 2, the reactor comprises an elongated, cylindrical reaction chamber 1 composed of a heat resistant material such as a suitable ceramic or metallic composition, chamber 1 being closed at its lower end by a removable closure plug 3 the upper end of chamber 1 having an opening 5 therein. The reaction chamber 1 is suspended and held in position within the well bore, usually at least several hundred feet from the earths surface, by tubing string 7 threadably engaged as at 9 with a heat resistant hanger ll embedded in the reaction chamber 1. If desned, the hanger 11 and reaction chamber 1 can be one integral unit. Entirely covering the inside and outside of the reaction chamber 1 is a thin film of a neutron absorbing material 13 which can advantageously be composed of zirconium having embedded therein particles of a neutron absorbing refractory material such as boron carbide. A useful neutron shielding material is described in the patent to Rockwell at al., US. No. 2,727,996.

Extending downwardly from the surface of the earth through the interior of tubing string 7 is rod 15 suitably connected at its lower end 17 with heat resistant rod 19 extending into reaction chamber 1 through opening 5. Rod 23 which is composed of a suitable concentration of a thermal neutron fissionable material such as 233 or U 235 is connected at the lower end of heat resistant rod 19 by any suitable means such as threads 21. Rod 15 is connected at the surface to a piston means hereinafter described for the purpose of raising or lowering the fissionable material 23 out of or into the lowermost portion of the reaction chamber 1.

The reaction chamber 1 is separated into an upper and an active lower portion by an annular partition 25 composed of a thermal neutron absorbing material. If desired this partition 25 can be of the same material as film 13. The active lower portion of chamber 1 contains an annular tube 27 composed of a thermal neutron fissionable material, e.g. 233. Tube 27 is spaced from the wall of reaction chamber 1 by heat resistant blocks 29 and held in position at its lowermost edge by retaining protuberance 31 of end closure 3. Spaced inwardly from tube 27 is a second annular tube 33 composed essentially of a neutron moderating material such as graphite or beryllium of sufficient thickness to slow the speed of the neutrons to about 8,000 feet per second. The annular moderating tube 33 is held in spaced relation to the tube of fissionable material 2'7 by extensions 35 of tube 33 at the top of tube 27 and by retaining protuberance 37 at its bottom. At any suitable position within the active lower portion of chamber 1 is a thermocouple located within the thermowell 39. The couple is connected to temperature control device 41 by wires in the line 43. The thermocouple current will give an indication at the earths surface of the temperature of the fission reaction. The thermocouple could, of course, be located outside of chamber 1.

Referring to the schematic view of the above surface control apparatus, it is seen that the reaction chamber 1 is in communication with the surface by tubing 7 and rod 15, the rod 15 being attached by suitable means to piston 45 operating within cylinder 47. The thermocouple within reaction chamber 1 is in electrical communication with temperature control device 41 by line 43, the temperature control device 41 being in electrical contact with a suitable solenoid control or similar device serving to activate pump means 51, which in turn operates piston 45 through a fluid, e.g., liquid medium thereby raising or lowering the rod 23 out of or into the active lower portion of chamber 1. At the mouth of the bore hole there is provided a layer of concrete 53 for holding the surface equipment.

The temperature controller is a known device and can be used to regulate the down hole temperature. For instance, when the temperature of the reaction goes below that desired, the thermocouple current decreases and the temperature controller signals pump 51 to withdraw fluid from beneath the piston which lowers rod 23 and intensifies the fission reaction to bring the temperature up to that desired. If the temperature be too high then the pump is actuated through the thermocouple and temperature controller to raise rod 23 through the passing of fluid from the pump to beneath piston 45 to decrease the intensity and temperature of the reaction. Other types of controls could be employed in place of those shown for varying the extent of contact of the fissionable material with the neutrons, for instance, the thermocouple indication could be visually noted and the pump controlled by hand to place the rod 23 in the desired position. The reactor of FIGURE 1 can advantageously be employed when providing temperatures of about 1000 to 2000 F.

As taught in the patent to Fermi et al., US. No. 2,708,- 656, a nuclear reactor to be self-sustaining must have fissionable material present in at least a critical mass and neutrons in high enough density to overcome any losses from the system. My reactors illustrated in FIGURES l to 5 operate by the simple expedient of increasing or decreasing the amount or concentration of the fissionable material within the reactor to a point above or below the critical mass of the system. By referring to FIGURE 1, it can readily be seen that if the fissionable element 27 is constructed so that it will contain less than the total critical mass of the system and if the combined masses of it and element 23 be at least equal to the critical mass of the system, the reactor will not become self-sustaining until the movable rod 23 is inserted into the active lower portion of the reaction chamber 1, a distance such that the mass of fissionable material in the lower portion of the chamber is at least equal to the critical amount. The annular partition 25 effectively blocks the passage of thermal neutrons between the fissionable elements when rod 23 is in the raised position.

The second factor in attaining a self-sustained nuclear reaction in the slow neutron reactor of FIGURE 1 is the presence of thermal neutrons. To accomplish this, I have positioned an annular tube of a neutron moderating substance 33, preferably graphite in such a manner that it will separate the annular tube of fissionable material 27 from the fissionable rod 23. By constructing the moderating tube 33 of sufficient thickness the high energy neutrons released by the fission in the separate elements 23 and 27 will be slowed down to the thermal energy of the system before contacting the fissionable nuclei of the opposite member. After the reaction chamber has been placed in position within the bore hole and the controls connected as hereinbefore described the rod 23 is lowered into the tube 33. As rod 23 enters the system the critical mass will be attained and the neutron density, showing an exponential rise, will build up to a point at which the reaction will become self-sustaining. As the rate of fission within the system rises the temperature will show a corresponding rise, thermocouple 39 transmitting the change in temperature to the surface temperature controlling device. The degree of heat generated within the reactor is controlled by raising and lowering the fissionable rod into or out of the graphite tube. When the temperature reaches the desired level the temperature control device can serve to locate the rod 23 through control of piston 45 as hereinbefore described.

In the embodiment of my invention shown in FIG- URES 3 to 5, I have illustrated a nuclear reactor which can be used advantageously combined with a conventional bore hole oil pump positioned within a cemented liner opposite an oil-bearing stratum. Referring to FIGURE 3, I have shown the nuclear reactor having a deuterium oxide moderating shield rather than graphite or beryllium. The reactor-pump unit comprises a cylindrical barrel 55 composed of two sections, the upper section 57 containing the oil pump 59 and the lower section 60 containing the reaction chamber, composed of barrel 62 and piston 61. The entire reactor-pump unit 55 is connected by cap 55a and tubing string 63 to surface equipment for elevating and lowering the tubing string and sucker rod 73, see for 5 instance FIGURE 1. The upper section 57 of unit 55 is composed of a suitable heat resistant material such as a ceramic or metallic composition having perforations 65 therein for the admission of oil. Contained within section 57 is the oil pump'59 composed of barrel 67 threadably attached as at 69 to cap 55a. Operating within barrel 67 is piston 71 connected to a suitable pumping means (not shown) at the surface by sucker rod 73. Inlet valve 75 is provided in barrel 67, and outlet valve 77 is provided in piston 71 for the passage of the oil through the pump. The pumping unit 57 is closed at the bottom by a suitable plate member '79 having opening 81 therein for the release of pressure in the reaction chamber as hereinafter described.

The lower section 60 of unit 55 is joined to the upper section 57 by any suitable means such as threads 83. The reaction chamber comprises an outer cylindrical barrel 62 and inner piston 61, both composed of a heat resistant material. Within barrel 62 is rigidly suspended a rod 85 composed of a fissionable material such as U 235. Rod 85is held in position within barrel 62 by any suitable suspending means such as by plate 87. Positioned in suspending means 87 is a pressure regulating valve 89. Barrel 62 is sealably and slidably engaged with piston 61 by O ring seals 91. The fissionable rod 85 is composed of two portions, the lowermost portion 93 containing the fissionable material and the uppermost portion 95 being composed of a heat resistant non-fissionable material. The lower portion 93 extends for such a length that when rod 85 is lowered into the heavy water, hereinafter described, the heavy water will completely cover the fissionable material 93.

The piston 61 contains an elongated cylindrical tank 97 whose bottom is formed by the threaded connection 101 between piston 61 and plug 105 and whose upper end is flared outwardly into an enlarged container 99 positioned in barrel 62 above piston 61. The upper flared portion 99 of tank 97 is partially closed at its top by guide 111. The tank is positioned at its upper end by spacing blocks 103 connected to piston 61. The piston 61 is closed at its lower end by closure plug 105 and the piston is maintained in sliding engagement with barrel 62 by ring seals 113.

Disposed within tank 97 is annular tube'107 composed of a thermal neutron fissionable material. Annular tube 107 is held in spaced relation to tank 97 by spacer protuberances 109. The anchoring device for holding the piston 61 in place within the well bore, can be constructed in any desired manner. In the embodiment of FIGURE 3, I have shown member 115, the jaws 117 of whichare attached to weight 121 by arms 116 pivotally connected to weight 121 at 119 and to their respective casing gripping jaw. Member 115 can be placed in the well and when the cone-shaped end fixture 125 on the lower end of piston 61 contacts the jaws, it divides and moves them to a holding position against the casing. So placed, memher 115 serves to provide a support on which the nuclear reactor can be located and operated.

Referring to FIGURES 4 and 5, the nuclear reactor of FIGURE 3 is shown with its parts in full operative engagement; however, the pump unit 57 is not illustrated. In FIGURE are shown the reaction chamber barrel 62, the piston 61, elongated tank 97, annular tube 107 of fissionable material and rod 85 of fissionable material. As seen in FIGURE 5, the annular tube of fissionable material 107 can be constructed so that its diameter will provide space A between it and the Wall of tank 97 which is approximately one-half the thickness of space B between it and rod 85 of fissionable material. Also, in FIGURE 5, I have shown a neutron reflecting layer of beryllium on the inside surface of tank 97; zirconium plating on the inside and outside surfaces of tube 107 and on the outside surface of rod portion 93; and a zirconium-steel alloy strengthening core in the center of rod portion 93. The heavy water between members 97 and 107 and between the rod 93 and tube 107 can serve to modify the speed of the neutrons from say, 10,000 miles per second to 7,000 to 8,000 feet per second.

As in the embodiment of FIGURES 1 and 2 the mass of the fissionable element 107 is less than the total critical mass; the fissionable masses of elements and 107, when combined, however, being at least equal to the critical mass of the system thus necessitating the insertion of rod 55 into annular tube 107 before the reactor will become self-sustaining. In this reactor, tank 97 is partially filled with deuterium oxide or heavy water which serves as a thermal neutron moderating material separating fissionable tube 107 from fissionable rod 85. In the operation of the reactor, barrel 62 can be slid downward to insert rod 35 into tube 107 to intensify the fission reaction and provide an increase in temperature. The temperature is decreased or the reaction stopped completely by withdrawing or removing rod 85 from tube 107. The movements of fissionable materials and the temperature can be controlled as described with reference to the apparatus of FIGURE 1. Since heavy water will boil at the elevated temperatures employed, super-atmospheric pressures can be maintained on the reaction chamber by pressure regulator 89 to keep the water in the liquid phase. The reactor of FIGURES 3-5 can advantageously be employed when providing temperatures approximating 400 F.

As mentioned before the central rod 85 is made up of two parts, a lower fissionable portion 93 and an upper non-fissionable portion made of a heat resistant material such as aluminum, zirconium, etc. The lower portion 93 of rod 85 is constructed in such a manner that when rod 85 is fully lowered into the reaction chamber as shown in FIGURE 4 the deuterium oxide will be displaced to a point above the juncture of the fissionable and nonfissionable portions of the rod 85. The heavy water above the fissionable portion which has been displaced into surplus water tank 99 will act as a thermal neutron reflecting material serving to lessen the loss of thermal neutrons from the system.

Referring now to the embodiment of FIGURE 6, I have shown a nuclear reactor of the thermal neutron or slow type having fixed fissionable rods, the total masses of the rods being at least equal to the critical mass of the system, and an elongated, rotatable neutron moderating plate for controlling the production of thermal neutrons and thus the rate of fission and the temperature within the reactor. The reaction chamber 131 comprises an elongated cylindrical member 132 composed of a suitable heat resistant material similar to that of FIGURES 1 through 5. Chamber 131 is closed at its lower end by plug 133 and at its top by header 135. Chamber 131 is positioned within the well bore 139 by tubing 137 suspended from the above-ground installation and connected to chamber 131 through header 1135. Within chamber 131 are positoned four fissionable rods 141, composed of a suitable thermal neutron fissionable material such as uranium-235 and the centrally positioned elongated rotatable neutron moderating plate 143. Rods 141 and plate 143 are held in position within chamber 131 by bottom plate 145 affixed to the reaction chamber wall in any suitable manner such as bolts 147 and upper plate 149, also aflixed to the reaction chamber wall by similar means. The lower plate 145 can have cup-like members 148 attached thereon for receiving the end of the fissionable rods and a bearing mount 150 upon which the moderating plate 143 rotates. The upper plate 149 has suitable clamps 152 for retaining the upper end of the fissionable rods and a suitable connecting linkage 154 by which the plate 143 is rotated.

The connecting linkage 154 can be placed in communication with the surface by rod 151 extending through the interior of tubing 137. Rod 151 and connecting linkage 154 are provided with suitable engaging slots 153. Rod 151 extends to the surface through bore hole plugging means 156 and is connected to a rotating device such as a hand wheel 155. A thermowell 57 containing a thermocouple is provided at any suitable position within chamber 131. The thermocouple, could, of course, be positioned outside the chamber if desired. The thermocouple is in electrical communication with an abovesurface temperature indicating device 159 by wires within line 161. At the mouth of the bore hole is provided a layer of concrete I63 for holding the surface equipment in position.

FIGURE 7 is a plan view taken through line 7-7 of FIGURE 6 showing the reaction chamber 131, four fissionable rods I41, elongated moderating plate 143 and fixed neutron inhibitor plates 165 positioned so as to partially separate the fissionable rods. As clearly seen in this figure the neutron moderating plate is comprised of an inner and an outer section. Inner section 167 can be composed of a suitable neutron shielding or blocking material, e.g., a material through which a substantial proportion of the neutrons are unable to penetrate. Such a material is described in U.S. Patent No. 2,727,996 to Rockwell et al. The outer section 169 can be composed of a suitable neutron moderating material such as graphite or beryllium. The fixed inhibitor plates 165 can advantageously be comprised of a material similar to that of the interior portion 167 of plate 143.

The elongated rotatable plate 143 will be constructed and positioned so that when plate 143 is rotated to the closed position it will come into alignment with fixed inhibitor plates 165. When in this position, the neutron inhibiting portion 167 of the plate 143 will form a continuous wall with the fixed inhibitor plates 165 extending across the interior of the reaction chamber thereby blocking the passage of a substantially proportion of neutrons through the neutron moderating material.

Since the temperature is directly proportional to the rate of fission and, therefore, the thermal neutron density in a system having a given critical mass, it is obvious that by changing the position of the moderating plate a change in thermal neutron density is eliected, thus increasing or decreasing the temperature as desired.

In operation, the reactor is placed in position at the desired depth within the bore hole with the rotatable plate in alignment with the fixed inhibitor plates and the thermocouple connected to the above-surface temperature indicating device. To start the nuclear reaction the moderating plate is slowly rotated by means of the hand wheel located at the surface. As the plate is rotated the neutron moderating portion of the plate will move into position between the fissionable rods on a given side of plates 165, thus causing a greater number of fast neutrons to contact this material resulting in a larger production of thermal neutrons and hence an increase in the rate of fission and ultimately the temperature; the thermocouple serving to indicate this rise in temperature. If the temperature rises above the desired point the moderating plate can be rotated toward its closed position, thus decreasing the area of neutron contact and subsequently the rate of fission. If the moderating plate is moved back into alignment with the fixed inhibitor plates, no neutrons will pass through the moderating material and into fissionable nuclei thereby effectively stopping the production of thermal neutrons and hence the fission.

Several modifications are obvious in this diagram, such as a substitution of an automatic temperature responsive mechanical rotating device rather than the visual temperature indicator and hand wheel or the substitution of a greater or lesser number of fissionable rods within the reaction chamber.

In some cases it may be desirable to equip this reactor with a means of stopping the reaction completely within a very short period of time in order to prevent runaways. If so, a control or safety rod 171 made of any material having a high degree of aflinity or absorbing capacity for neutrons, such as boron or cadmium, can be arranged to fall under the force of gravity into C) the interior of the reaction chamber. One or more control rods can he insertable from the earths surface into the chamber through an opening in header I35 and top plate 149 and held in inoperative position by a piston I73 and pump means located at the earths surface. Such means could be similar to that described in conjunction with FIGURE 1. When the temperature responsive device shows a dangerously rapid increase in temperature, indicating a dangerous rise in neutron densities, an electrical impulse will be communicated to the pump by wires 177 causing fiuid to be withdrawn from beneath the piston lowering the control rod into the chamber and thereby bring about the absorption of a large number of neutrons, thus removing them from the system and reducing the neutron density to a point below the danger level. In order to start the reaction, fluid would be pumped below the piston causing the safety rod to be removed from the chamber.

Another method of control previously mentioned is by varying an amount of a neutron absorbing material Within the reaction chamber containing the critical mass of the system. This could be done very easily in FIGURE 6 by substituting an absorbing rod of boron or like material in the place of moderating plate 143 and providing means of raising or lowering this rod out of or into the chamber. Rod and piston means similar to that described in reference to FIGURE 1 could be used to accomplish this purpose if desired. In a reactor of this type the neutron moderator could be introduced into the reaction chamber as a liquid such as deuterium oxide. Alternatively, of course, the neutron moderator could be shaped pieces of solid graphite geometrically spaced around the fissionable rods. If the moderating medium is deuterium oxide a pressure regulating valve could be positioned on plate 149 to retain the deuterium oxide in the liquid phase at the elevated temperatures. By lowering the control rod into the chamber to differing depths in response to a temperature control or similar device, the neutron density will be varied thereby increasing or decreasing, as the case may be, the rate of fission and the temperature.

Although the reactor of FIGURE 3 has been incorporated with an oil pump, it is obvious that this pump unit can be omitted and in no manner hinder the operation of my apparatus. In my nuclear reactors, a suitable source of neutrons can be initially provided and the source can be relied upon to maintain a minimum density of neutrons when they are not provided through operation of the reactors. As an example, US. Patent No. 2,440,999 described a compressed neutron source which can be located on member 37 of FIGURE 1; in the bottom of tube 107 of FIGURE 4 or around the inside of the periphery of the reactor of FIGURE 6. If desirable, the fissionable rods can be provided with a strengthening core of heat resistant material such as a zirconium-steel alloy and an outer sheath of a similar material as shown in FIGURE 5. Likewise, the tubular fissionable elements can be strengthened with a heat resistant metallic sheath. Also, the reactor can be covered with a material to decrease gamma ray radiation. When the fissionable elements within the reactor have decomposed into their lower molecular weight components the reactor can be brought to the surface and new fissionable elements inserted therein, the radio-active waste materials being disposed of in any suitable fashion.

I claim:

1. An apparatus for heating a subsurface oil-bearing stratum which comprises an unshielded nuclear reactor positioned in a well bore opposite to and in heat relationship to an oil-bearing stratum with said oil-bearing stratum acting as a shield and coolant for the reactor and with the heat developed in said oil-bearing stratum being sutlicient to cause the oil in said stratum to flow, said nuclear reactor including two separate portions of fissionable material, a first portion of which is below the critical mass needed to maintain a controlled, selfsustained nuclear reaction, a source of thermal neutrons in contact with a neutron moderating substance and said first portion of fissionable material, means at the earths surface for controlling the intensity of the fission reaction by varying the amount of a second portion of fissionable material in contact with moderated thermal neutrons and thereby controlling the heat of said stratum, the amount of said first and second portions of fissionable material in contact with the moderated thermal neutrons being sufficient to provide at least said critical mass.

2. An apparatus for heating a subsurface oil-bearing stratum which comprises an elongated nuclear reaction chamber composed of a heat-resistant material, said reaction chamber being suspended from the surface of the earth within a well bore traversing the oil-bearing stratum and being opposite said stratum, a tube of fissionable material positioned in said reaction chamber and containing less than the critical mass necessary to produce a controlled, self-sustained nuclear reaction, a second portion of fissionable material of sufficient diameter to allow its insertion into said tube of fissionable material, a source of thermal neutrons in contact with said tube of fissionable material, a neutron moderating substance in contact with said thermal neutrons, means at the earths surface for lowering and raising said second portion of fissionable material within the tube of fissionable material to vary the intensity of the nuclear reaction when the amount of fissionable material in contact with moderated thermal neutrons is at least equal to the critical mass needed to maintain a controlled, self-sustained nuclear reaction, and means to indicate at the earths surface the temperature of the fission reaction.

3. An apparatus for heating a subsurface oil-bearing stratum which comprises an elongated nuclear reaction chamber composed of a heat-resistant material, said reaction chamber being suspended from the surface of the earth within a well bore traversing the oil-bearing stratum and being opposite said stratum, an annular partition composed of a neutron absorbing material dividing said reaction chamber into upper and lower portions, a tube of fissionable material positioned in the lower portion of said reaction chamber and containing less than the critical mass necessary to produce a controlled, self-sustained nuclear reaction, a second portion of fissionable material of sufficient diameter to allow its insertion into said tube of fissionable material, a source of thermal neutrons in contact with said tube of fissionable material, a tubular neutron moderating substance in contact with said thermal neutrons and spaced inwardly of said tube of fissionable material, means at the earths surface for lowering and raising said second portion of fissionable material from the upper portion of the reaction chamber through the annulus of the partition to within the tubular moderating substance to vary the intensity of the nuclear reaction when the amount of fissionable material in contact with moderated thermal neutrons is at least equal to the critical mass needed to maintain a controlled, self-sustained nuclear reaction.

4,. An apparatus for heating an oil well bore hole and a surrounding oil-bearing stratum, which comprises a nuclear reactor positioned in a well bore opposite said stratum, said nuclear reactor having a tubular member composed of a heat-resistant material, a first portion of fissionable material in said tubular member, a thermal neutron moderator container slidably held within said tubular member, a second portion of fissionable material within said container, a neutron moderating material in said container, said second portion of fissionable material containing less than the total critical mass necessary to produce a controlled, self-sustained nuclear reaction, a source of thermal neutrons in said container for contact ing said fissionable material, means at the earths surface for lowering and raising said first portion of fissionable material within the container to vary the intensity of the fission reaction when the amount of fissionable material in contact with moderated thermal neutrons is at least equal to said critical mass, and means to indicate at the earths surface the temperature of the fission reaction.

5. An apparatus for heating an oil well bore hole and a surrounding oil-bearing stratum, which comprises a nuclear reactor positioned in a well bore opposite said stratum, said nuclear reactor having a tubular member composed of a heat-resistant material, a first portion of fissionable material rigidly suspended within said tubular member, a thermal neutron moderator container slidably held within said tubular member, a tube of fissionable material within said container, a liquid neutron moderating material in said container, said tube of fissionable material being of sufiicient diameter to allow said first portion of fissionable material to be inserted therein, said tube of fissionable material containing less than the total critical mass necessary to produce a controller, self-sustained nuclear reaction, a source of thermal neutrons in said container for contacting said fissionable material, means at the earths surface for lowering and raising said first portion of fissionable material within the tube of fissionable material to vary the intensity of the fission reaction when the amount of fissionable material in contact with moderated thermal neutrons is at least equal to said critical mass.

References Cited in the file of this patent UNITED STATES PATENTS 2,632,836 Ackley Mar. 24, 1953 2,743,224 Ohlinger Apr. 24, 1956 2,769,096 Frey Oct. 30, 1956 2,778,950 Frey et a1 Jan. 22, 1957 2,854,584 Youmans Sept. 30, 1958 FOREIGN PATENTS 999,330 France July 16, 1952 OTHER REFERENCES Stephenson, R.: Introduction to Nuclear Engineering, McGraw-Hill Book Co., New York (1954), pp. 282, 294.

Proceedings of the International Conference on the Peaceful Uses of Atomic Energy, vol. II, held in Geneva, August 8-20, 1955, United Nations, New York, 1956, pp. 435-436. 

1. AN APPARATUS FOR HEATING A SUBSURFACE OIL-BEARING STRATUM WHICH COMPRISES AN UNSHIELDED NUCLEAR REACTOR POSITIONED IN A WELL BORE OPPOSITE TO AND IN HEAT RELATIONSHIP TO AN OIL-BEARING STRATUM WITH SAID OIL-BEARING STRATUM ACTING AS A SHIELD AND COOLANT FOR THE REACTOR AND WITH THE HEAT DEVELOPED IN SAID OIL-BEARING STRATUM BEING SUFFICIENT TO CAUSE THE OIL IN SAID STRATUM TO FLOW, SAID NUCLEAR REACTOR INCLUDING TWO SEPARATE PORTION OF FISSIONABLE MATERIAL, A FIRST PORTION OF WHICH IS BELOW THE CRITICAL MASS NEEDED TO MAINTAIN A CONTROLLED, SELFSUSTAINED NUCLEAR REACTION, A SOURCE OF THERMAL NEUTRONS IN CONTACT WITH A NEUTRON MODERATING SUBSTANCE AND 